A good spellchecker - http://www.hotlingo.com/

The evaluation download version lasts as long as you want it, and the FAQ on the site says it's fine not to pay anything, so long as you recommend it to other people. Which I do.

Like any spellchecker, it's not just for mispellings, it picks up the mis-typings as well. For me that is a lot more important, because I am very prone to doing those.